The following wiki article details setting up an Outbound Gateway for mail routing: http://www-10.lotus.com/ldd/bhwiki.nsf/dx/Routing_outbound_email
The article details setting up a single gateway, is it possible to setup a back-up Outbound Gateway for mail routing?
Answer: Adding multiple IP's are not supported for the Outbound Gateway settings. However, the Outbound Gateway value can set as a DNS host name. The DNS record can be setup to point to multiple server IP addresses. If there is any Outbound Gateway failure, then the failed server IP can be removed from the DNS record and the gateway will continue to process mail.